[PATCH 0/3][V2] remove the ns_cgroup

Daniel Lezcano daniel.lezcano at free.fr
Mon Sep 27 13:43:18 PDT 2010

On 09/27/2010 09:57 PM, Andrew Morton wrote:
> On Mon, 27 Sep 2010 12:14:10 +0200
> Daniel Lezcano<daniel.lezcano at free.fr>  wrote:
>> The ns_cgroup is a control group interacting with the namespaces.
>> When a new namespace is created, a corresponding cgroup is
>> automatically created too. The cgroup name is the pid of the process
>> who did 'unshare' or the child of 'clone'.
>> This cgroup is tied with the namespace because it prevents a
>> process to escape the control group and use the post_clone callback,
>> so the child cgroup inherits the values of the parent cgroup.
>> Unfortunately, the more we use this cgroup and the more we are facing
>> problems with it:
>>   (1) when a process unshares, the cgroup name may conflict with a previous
>>   cgroup with the same pid, so unshare or clone return -EEXIST
>>   (2) the cgroup creation is out of control because there may have an
>>   application creating several namespaces where the system will automatically
>>   create several cgroups in his back and let them on the cgroupfs (eg. a vrf
>>   based on the network namespace).
>>   (3) the mix of (1) and (2) force an administrator to regularly check and
>>   clean these cgroups.
>> This patchset removes the ns_cgroup by adding a new flag to the cgroup
>> and the cgroupfs mount option. It enables the copy of the parent cgroup
>> when a child cgroup is created. We can then safely remove the ns_cgroup as
>> this flag brings a compatibility. We have now to manually create and add the
>> task to a cgroup, which is consistent with the cgroup framework.
> So this is a non-backward-compatible userspace-visible change?
> What are the implications of this?

An application will have to create a directory in the cgroup directory 
and write the pid in the tasks file, instead of assuming it is 
automatically created with the unshare/clone. The cgroupfs should be 
mounted with the 'clone_children' option set.

AFAIK, I am the only one, with the lxc tools, using the ns_cgroup and I 
will be happy to get rid of it. People is used to change the default 
cgroup mount options to mount all the subsystems except the ns_cgroup 
(for example this is needed for libvirt if I am not wrong). IMHO, a very 
few people will be impacted, to not say nobody.

   -- Daniel

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